The face-down soldering of integrated circuit (IC) devices to chip carriers, known as “flip-chip packaging” has been used in the semiconductor manufacturing process for forty years. Injection Molding Soldering (IMS) is a technique developed by International Business Machines Corporation (IBM) to address the cost vs. quality issues associated with current wafer bumping technologies. IMS as applied to wafer bumping has been dubbed C4NP (Controlled Collapse Chip Connection New Process) and is a recent semiconductor packaging technology developed by IBM for putting C4 solder bumps onto chips at the wafer level using a lead-free solder. This process is sometimes known as “C4 wafer bumping.” C4NP involves filling specially designed pits in a solder mold (mold plate) using a fill head which injects molten solder onto the mold plate.
The multiple etched cavities on a mold plate match the pattern of solder bumps (the “bond pads”) required on a wafer. The cavity size and spacing determine the solder bump volume and pitch. The cavities give precise control of bump solder volume, resulting in excellent bump height uniformity. Typical applications might call for 75-μm diameter bumps on a 150-μm pitch. Smaller bumps down to 25-μm in diameter on a 50-μm pitch have been demonstrated, matching the fine-pitch capability of electroplated bumps. For a higher board-to-die standoff, jumbo BGA solder bumps could be molded to 500-μm diameter.
The mold plate is typically constructed of borosilicate glass. In selecting a material for the mold plate the Coefficient of Thermal Expansion (CTE) must be taken into consideration. For example, the thermal expansion of the glass mold matches the thermal expansion of the silicon wafer. Other materials may be used, provided the CTE is taken into consideration. The mold plate is heated to just below the melting point of the solder. The solder is typically constrained to the head and operating area by an o-ring that is compressed between the fill head and the mold plate.
Molten solder is injected into the mold plate cavities through a dispenser. The injector includes a slightly pressurized reservoir of molten solder of any composition. The filling head automatically fills all cavities in the mold, maintaining close contact with the mold plate. The cavities are uniformly filled with the molten solder to the level of the mold plate surface. Once the mold plate is filled the solder is transferred to create C4 bumps on a wafer.
In creating the solder bumps, often the fill head is secured to a moving arm which moves over and above a mold plate carrier (a belt or platform) where the mold plates are positioned and ready for filling. The fill head moves in one direction. Alternatively, the fill head remains stationary and it is the mold plate carrier which moves in one direction. The fill head remains stationary but it is considered to exhibit “relative” movement, rather than actual movement, with respect to the mold plate carrier.
The fill head must make contact with the mold plate to prevent solder leakage, yet it must provide a small gap to allow for air to escape. Pressing down too hard on the mold plate prevents solder leakage, but does not allow for air to escape which may result in deformation of the solder balls.
There is a need for an improved method of C4 wafer bumping to control the gap between the fill head and the mold plate in order to overcome the shortcomings of the prior art.